The invention relates to a charged particle apparatus comprising a particle source for generating a particle beam irradiating a specimen to be located in the lens field space of a lens which forms part of a charged particle lens system of the apparatus.
Such an apparatus in the shape of an electron beam apparatus is known from U.S. Pat. No. 4,306,149. This document discloses an electron microscope in which an object is located in a lens field space of an objective lens. This lens is provided with an auxiliary lens to enable easy switching between different modes of operation. In U.S. Pat. No. 4,820,898 an ion beam apparatus for which the invention is applicable is disclosed.
If in such an apparatus electrically non-conductive specimens are examined the specimen may be charged up. This charging-up of the specimen may result in additional electric fields in the volume around the specimen which may have adverse effects on the imaging properties of the objective lens and hence on the image quality. This drawback has been neglected or accepted in the past because due to the moderate vacuum in the apparatus it rapidly vanished as a consequence of contamination on the surface of the specimen. The surface becomes electrically conductive, and the charging-up vanishes. No possibility of avoiding contamination apart from heating up the specimen is known. Unfortunately carbon deposits on the specimen surface are also liable to impair the image quality oz to detoriate right landing positions in a beam writing system such an electron beam pattern generator or ion beam implantating apparatus.
In more modern instruments in which a relatively high vacuum can be realized which is necessity for studies of biological specimens etc., no substantial contamination occurs, and the phenomena of charging-up occur with all their drawbacks.
One known method of neutralizing the specimen charge consists in producing slow charged particles of opposite sign from an auxiliary electron or ion source The slow charged particles will then be attracted to the specimen by the electrostatic field of the specimen charge and thus neutralize it. This method is, however, not applicable if the specimen is located in a region with a strong magnetic field because, according to Busch theorem, slow particles cannot enter a strong magnetic field unless they start from the auxiliary source with specific initial conditions giving them corresponding initial values of the angular momentum about the optical axis. If the value of the angular momentum of a charged particle is outside a certain limited range of values, then the particle is rejected by a strong magnetic field ("magnetic mirror"). This restriction of the range of initial values of the angular momentum of the charged particles emitted from an auxiliary source is of great practical importance if the charged particles are electrons. On the other hand, if the charged particles are ions, then they can, due to their large mass, reach the specimen with almost any practically realizable value of their initial angular momentum. A neutralization of the specimen charge by ions would, however, have the drawback that their presence would change the composition of the specimen to be examined.